Method for making improved sealability polyolefin fibers, the fibers made thereby and non-woven textile materials including the fibers

ABSTRACT

A method for making improved sealability polyolefin fibers comprising extruding a polyolefin ( 2 ) and at least one polyolefin copolymer ( 4 ) having a melting temperature less than that of the melting temperature of the polyolefin to form fibers ( 9 ) having outer surfaces wherein at least part of the outer surfaces is comprised of the low-melting polymer and which have high sealability properties.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. application Ser. No. 09/449,585, filed Nov. 29, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates to a method for making improved sealability polyolefin fibers, the fibers made thereby, and non-woven textile materials including the fibers.

BACKGROUND OF THE INVENTION

[0003] Non-woven textile materials are conventionally made starting from fibers which are caused to adhere to one another by subjecting the fibers to a thermal process performed on a heated calendering apparatus, or in hot air ovens or the like.

[0004] Known fibers used for making non-woven fabrics have the main disadvantage of requiring a comparatively high temperature for mutually sealing the fibers. The sealing operation, in particular, involves a high power cost as well as processing difficulties. The processing difficulties mainly depend upon the fact that, because of the thermal processing, the fibers tend to degrade and to lose the desired mechanical properties.

SUMMARY OF THE INVENTION

[0005] Accordingly, it is a main object of the present invention to provide a novel method for making improved sealability polyolefin fibers. In particular, it is an object of the present invention to make improved sealability polyolefin fibers having melting temperatures less than the melting temperatures of known fibers that are typically used, for example, in non-woven fabrics.

[0006] It is another object of the present invention to provide improved sealability among the fibers regardless of the form of the fibers. The fibers, for example, may be in the form of cut fiber or in the form of filament (continuous fiber). Improved sealability means that the bond created among these fibers, for example in a conventional carded or spun-laid web, is greater where the web is heated on a calendaring apparatus or in a hot air oven or the like, then it would be otherwise.

[0007] It is yet another object of the present invention to provide a non-woven textile material that has very good strength properties and that can be used, for example, as cover stock liners in hygiene and sanitary products such as baby diapers, feminine napkins, adult incontinent products and others, wherein stringent levels of tensile strength are required.

[0008] One embodiment of the present invention which achieves the above mentioned objects, as well as other objects which will become more apparent hereinafter, is the method of the present invention which allows one to easily produce polyolefin fibers suitable for making a non-woven textile material having very high mechanical strength properties such as tensile strength, while maintaining softness and other important properties such as absorbency, durability, liquid acquisition, liquid distribution and uniformity.

[0009] Another embodiment of the present invention which also achieves the above mentioned objects as well as other objects is the fibers made by the method of the present invention. Said fibers have very good sealing properties. For example, the carded or spunlaid web realized with the fibers of the present invention can be processed on a heated calendaring apparatus or in a hot air oven or the like, at lower temperature conditions than conventionally used with other polyolefin fibers. This has the advantage of resulting in a high power savings.

[0010] Yet another embodiment of the present invention which also achieves the above mentioned objects as well as other objects is the non-woven textile material comprising fibers made by the method of the present invention. The non-woven textile material comprises a plurality of mutually sealed filaments wherein the filaments comprise fibers having an outer surface and a heterogeneous structure comprising polyolefin and at least one polyolefin copolymer having a melting temperature less than the melting temperature of the polyolefin and wherein the low-melting polyolefin copolymer comprises at least a portion of the outer surface of the fibers.

[0011] Other advantageous and preferred embodiments of the present invention are set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above objects and advantages as well as other objects and advantages of the present invention will become more apparent hereinafter from the following detailed disclosure of some preferred embodiments of the method and system of the present invention which are illustrated by way of a non-limiting example in the figures of the accompanying drawings.

[0013]FIG. 1 is a schematic view illustrating a system for making improved sealability fibers according to the present invention;

[0014]FIG. 2 is a cross-sectional view illustrating a fiber made by the system shown in FIG. 1;

[0015]FIG. 3 illustrates an exemplary non-woven textile material made by using the fibers shown in FIG. 2;

[0016]FIG. 4 is a perspective view illustrating, on an enlarged scale, two fibers of the textile material of FIG. 3;

[0017]FIG. 5 is a detailed view of a mutual sealing section of the fibers of FIG. 4;

[0018]FIG. 6 is a cross-sectional view illustrating a sheath-core type of fiber, according to the present invention;

[0019]FIG. 7 is a further cross-sectional view, illustrating a side-by-side type of fiber according to the present invention;

[0020]FIG. 8 is a graph of the load (tensile strength) in the machine direction (MD) measured in units of N/5 cm versus percent elongation, wherein measurements are made on a non-woven, 5 cm×25 cm fabric sample having a weight of 19.5 g/m² and a grip distance of 20 cm on dynamometer clamping; and

[0021]FIG. 9 is a graph of load (tensile strength) in the cross direction (CD) measured in units of N/5 cm versus percent elongation, wherein measurements are made on a non-woven, 5 cm×25 cm fabric sample having a weight of 19.5 g/m² and a grip distance of 20 cm on dynamometer clamping.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Generally, in order to evaluate the sealability of a polyolefin fiber, a non-woven fabric is prepared from the polyolefin fiber in the form of a web (carded from staple fibers or spunlaid web from filaments) and then the web is consolidated by calendaring under certain conditions of temperature and pressure. The fiber of the web may be in the form of filament fiber or cut fiber. Preferably the cut fiber is at least 38 mm in length, more preferably at least 40 mm in length. Subsequently, the load needed to break the non-woven fabric both in the direction parallel (machine direction (MD)) to and traverse (cross direction (CD)) to the calendaring is measured on the non-woven fabric sample. The maximum load value determined in this way is considered to be a measure of the fibers' sealability. This method is hereinafter referred to as “Method A.”

[0023] Another, more direct method to evaluate the sealability characteristics of a fiber is set forth below. Specimens are prepared from roving that is comprised of continuous filaments. The roving is twisted and the two extremities are united, thus, obtaining a product where the two halves of the roving are entwined as in a rope. The sealing is carried out on the specimen using a thermowelding machine such as a BRUGGEL HSC-ETL operating at a specified plate temperature and a specified clamping pressure for a given period of time for sealing to occur. A dynamometer is used to test the average force required to separate the two halves of the roving which constitute each specimen at the thermowelding point. The result, expressed in Newtons (N), is obtained by averaging out the measurements taken and represents the sealing strength of the fiber. Preferably, at least 8 measurements are taken. This method is hereinafter referred to as “Method B.”

[0024] Therefore, “sealability” is expressed as either the tension or load, measured in units of force per unit length such as N/5 cm, needed to tear a specimen of a non-woven fabric prepared from the fiber as in Method A or the force in Newtons (N) to separate two halves of the roving comprised of fiber in the form of continuous filament which constitutes each specimen at the thermowelding point as in Method B.

[0025] Elongation, as illustrated in FIGS. 8 and 9, is expressed as a percentage (%) calculated between the initial length of the non-woven sample and the actual length of the sample under tension and finally at breaking.

[0026] The system shown in FIG. 1 comprises a first extruder 1, which is located with a set amount of molten homopolymeric material 2, as well as a second extruder 3, provided for supplying a molten polyolefin copolymer material 4 having a melting temperature less than the melting temperature of the polyolefin material 2. The term “polyolefin” as used herein refers to a class of thermoplastic polymers derived from simple olefins. Advantageously, the preferred copolymers would be constituted by copolymers having a melting temperature of about 140-150° C. Preferred polyolefins are polyethylene, polypropylene, polybutenes, and polyisoprene and their copolymers. Polypropylene is particularly preferred for the material 2 and propylene copolymer is particularly preferred for the material 4. Polypropylene homopolymer in the form of pellets having melt flow rate (M.F.R.) of 11 g/10 minutes and melting point of 160° C. is more particularly preferred for the material 2 and a random propylene copolymer which is available from MONTELL COMPANY (ITALY) in the form of pellets as CLYRELL™ 721RCXP having melt flow rate of 10.7 g/10 minutes and melting point of 148° C. is more particularly preferred for the material 4.

[0027] The polyolefin 2 and the low melting copolymer 4 are supplied to a mixer 5 from the outlet of which a melt conglomerated mass 6 having a substantially heterogeneous structure of the two materials 2 and 4 is supplied.

[0028] The mass 6 is sent to a die assembly 7 of a conventional apparatus for making short-spinning fibers wherein filaments 9 are ejected by the die hole 8.

[0029] The filaments 7, as seen in cross section as shown in FIG. 2, present a heterogeneous structure or composition constituted by granules or portions of low melting copolymer 4, dispersed through the high melting polyolefin 2.

[0030] Thus, by the disclosed method, and using a polyolefin-polyolefin copolymer mixture, the polyolefin copolymer being preferably included in an amount of 20-30% by weight with respect to the total polymeric mass, fibers having a count of 2.2 dtex are obtained. The evaluation of the sealability of these fibers is carried out by a specimen of filament roving on the thermowelding machine at 148° C. temperature (i.e. the melting temperature of the polyolefin copolymer 4). The result is a sealing strength of the fibers of 8-10 N measured according to the above identified method “B.” The sealability is created only between the peripheral portions where the low melting copolymer 4 is existing and not between the peripheral zones where the high melting polymer exists. This means that the fibers of the present invention are converted by carding or spunlaying in webs to be calendared now at temperatures lower than 8-10° C. in comparison with the temperatures required for webs realized in 100% in polyolefin high melting homopolymers fibers.

[0031] Actually, as is shown in FIGS. 3 to 5, the non-woven textile material or fabric 15 is made by calendering the fibers 9, or by a like method, as a high strength owing to the mutual sealing of the fibers 9 at the level of the low melting copolymer 4 portions appearing on the surface of the fibers.

[0032] The remaining mass of polyolefin 2, melting at a temperature greater than that used in the calendering process, is held unaltered, thereby providing the non-woven textile material with very good mechanical properties, typical of fibers made exclusively of polyolefin.

[0033] In the variation shown in FIG. 6, the polyolefin material 2 and polyolefin copolymer 4 are processed for providing bicomponent fibers, and, more specifically, sheath-core fibers.

[0034] In this case, the filament 12 comprises a polyolefin core 2, encompassed or sheathed by a low melting copolymer 4 outer sheath. Thus, by this method, carried out starting from a polyolefin copolymer having a melting temperature of 148° C. and in an amount of 20-30% by weight based on the total polymeric mass weight, sheath-core fibers in a count of 2.2 dtex or any other fiber in a count range from 1 to 70 dtex. Particularly, the sheath-core fiber in the count of 2.2 dtex has a very high sealability measured according to the evaluation method “B” as above identified; the sealability of the 2.2 dtex fiber is 10-12 N. These results, in turn, cause an increase or improvement of the toughness and strength properties C.D. (cross direction) elongation and M.D. (machine direction) ultimate elongation of the non-woven fabric or textile material made by the fibers.

[0035] Actually, in the case of a non-woven textile material of 19.5 g/m², with a calendering speed of 200 m/min and at a sealing temperature of 147-149° C., the following sealability values were obtained: 39.11 N/5 cm in the M.D. and 9.56 N/5 cm in the C.D., with respective ultimate elongations of 44.87% and 86.55%. These values would be indicative of a very good product, according to present standards (see FIGS. 8 and 9, respectively).

[0036] The same disclosed method can also be advantageously used for making fibers of 0.5 to 30 dtex values.

[0037] According to a modified embodiment shown in FIG. 7, the filaments 14 have a structure in which the component providing the sealing of the fibers consists of the low-melting copolymer portion 4 thereof.

[0038] The invention, as disclosed and illustrated, is susceptible to several modifications and variations all of which will come within the scope of the accompanying claims.

[0039] Thus, for example, the fibers could also be provided with a multiple component construction, of any desired configuration, including two different low melting copolymers, in combination with a set amount of polyolefin.

[0040] Moreover, instead of the two disclosed extruders, it would be also possible to use a single extruder, including one or more screws for processing simultaneously the two copolymers.

[0041] In this connection, it is desired to point out that an amount of 100% low melting (148° C.) polyolefin copolymer has been also processed, but the obtained results have not provided the desired sealability properties, since the fiber softened through the overall cross-section thereof, thereby losing its mechanical strength properties.

[0042] The invention is further illustrated by the following non-limiting example.

EXAMPLE 1

[0043] Specimens are prepared from 500 tex roving (ASTM D 1577-7) that is 40 cm in length and comprised of continuous filaments. The roving was twisted two times per centimeter and the two extremities were united to obtain a product wherein the two halves of the roving were entwined as in a rope. The sealing was carried out on said specimen using a thermowelding machine (BRUGGEL HSC-ETK) operating at plate temperature of 150° C. using a clamping pressure of 40 psi and 1 second of sealing time. A dynamometer was used to test the average force required to separate the two halves of the roving which constituted each specimen at the thermowelding point. The result, expressed in Newtons (N/5 cm), was obtained by averaging out at least eight measurements and represented the sealing strength of the fiber. This was an example of Method B.

EXAMPLE 2

[0044] A sealable polyolefin of 2.2 dtex in a cut length of 40 mm was prepared in a conventional short-spinning compact line. The two components of the fiber were subjected to a sheath-core type melt spinning according to U.S. Pat. No. 5,869,106 resulting in a bundle of filaments. A spin finish oil was applied to the bundle of filaments. The bundle of filaments was drawn in a draw ratio of 1:1.8. The drawn bundle of filaments was crimped in a stuffer box crimper. The drawn and crimped bundle of filaments were heat set, and the fiber was cut in 40 mm cut length to be put in bales.

[0045] The sheath material consisted of a random propylene copolymer available from MONTELL COMPANY as CLYRELL™ 721RCX having a M.F.R. of 11.7 g/10 minutes and a melting point of 143° C. The core material consisted of a polypropylene homopolymer available from MONTELL COMPANY under the trademark MOPLEN F30S having a M.F.R. of 11 g/10 minutes. The extrusion of the bundle of filaments was at a temperature of about 300° C. The sealability was tested on a specimen of the fiber filaments in a roving of 500 tex being 2.2 dtex of the single filament. The sealability was 12 N. A non-woven fabric was also prepared using the obtained staple sealable fiber in 2.2 dtex, cut length 40 mm by preparing a carded web of a weight of 19.5 g/m². It was subsequently hot calendared at a temperature from 147° C. up to 149° C. and at a speed of 200 meters/minute. Fabric samples were tested according to the tensile strength test method identified above as Method B. The tensile strength was measured on the non-woven fabrics sample according to the EDANA recommended method 20.02.1989 or the equivalent test method DIN 53857 or ASTM D 1682 and the breaking tensile strength values on the samples of the non-woven fabrics were recorded. The maximum load and percentage of elongation at breaking, as illustrated by the curve of FIGS. 8 and 9, were as follows:

[0046] maximum load in machine direction (MD) was 39.11 N/5 cm;

[0047] maximum load in cross direction (CD) was 9.56 N/5 cm;

[0048] percent elongation for MD was 44.87%;

[0049] percent elongation for CD was 86.85%.

[0050] It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof. 

I claim:
 1. A method for making an improved sealability polyolefin fiber comprising extruding a polyolefin and at least one low-melting polyolefin copolymer having a melting temperature of about 140-150° C., to form a polyolefin fiber wherein the polyolefin fiber has one or more outer surfaces comprising the low-melting polyolefin copolymer, and wherein the polyolefin fiber is in a form of a filament or a cut fiber having a cut length of at least 38 mm, and wherein the polyolefin fiber has a count of 1 to 70 dtex.
 2. The method according to claim 1 , wherein the method comprises extruding a mixture of molten masses of polyolefin and low-melting polyolefin copolymer to provide a heterogeneous structure fiber comprising the polyolefin and the low-melting polyolefin copolymer.
 3. The method according to claim 1 , wherein the low-melting polyolefin copolymer has a melting temperature of 148° C.
 4. The method according to claim 1 , wherein the method comprises extruding the polyolefin and the low-melting polyolefin copolymer in a sheath-core extrusion process.
 5. The method according to claim 1 , wherein the method comprises extruding the polyolefin and the low-melting polyolefin copolymer in a side-by-side extrusion process.
 6. The method according to claim 1 , comprising extruding the polyolefin and the low-melting polyolefin copolymer to form a fiber having a composite construction and geometry.
 7. The method according to claim 1 , wherein the polyolefin is polyethylene, polypropylene, polybutene, or polyisoprene.
 8. An improved sealability polyolefin fiber having an outer surface, wherein the polyolefin fiber is in the form of a filament or a cut fiber having a cut length of at least 38 mm and wherein the polyolefin fiber has a count of 1 to 70 dtex and wherein the polyolefin fiber has a heterogeneous structure comprising polyolefin and at least one low-melting polyolefin copolymer wherein the low-melting polyolefin copolymer has a melting temperature of about 140-150° C. and wherein the low-melting polyolefin copolymer comprises at least a portion of the outer surface of the polyolefin fiber.
 9. The polyolefin fiber according to claim 8 , wherein the polyolefin fiber is a bicomponent sheath-core fiber having an outer sheath comprising the low-melting polyolefin copolymer and a core comprising the polyolefin.
 10. The polyolefin fiber according to claim 8 , wherein the low-melting polyolefin copolymer has a melting temperature of 148° C.
 11. The polyolefin fiber according to claim 8 , wherein the polyolefin fiber is a side-by-side fiber comprising both the polyolefin and low-melting polyolefin copolymer.
 12. The polyolefin fiber according to claim 8 , wherein the fiber has a sealability of 10 to 12 N for 2.2 dtex fibers.
 13. The polyolefin fiber according to claim 8 , wherein the polyolefin is polyethylene, polypropylene, polybutene, or polyisoprene.
 14. A non-woven textile material comprising a web wherein the web is comprised of polyolefin fiber in a form of a filament or a cut fiber having a cut length of at least 38 mm and wherein the polyolefin fiber has a count of 1 to 70 dtex and wherein the polyolefin fiber has an outer surface and a heterogeneous structure comprising polyolefin and at least one polyolefin copolymer having a melting temperature of about 140 -150° C. and wherein the low-melting polyolefin copolymer comprises at least a portion of the outer surface of the polyolefin fiber.
 15. The material according to claim 14 , wherein the low-melting polyolefin copolymer has a melting temperature of 148° C.
 16. The material according to claim 14 , wherein the web comprises a bicomponent sheath-core fiber having an outer sheath comprising the low-melting polyolefin copolymer and a core comprising the polyolefin.
 17. The material according to claim 14 , wherein the web comprises a side-by-side fiber comprising both the polyolefin and low-melting polyolefin copolymer.
 18. The material according to claim 14 , wherein filaments are sealed to one another at that portion of the outer surface of the fibers formed by the low-melting polyolefin copolymer.
 19. The material according to claim 14 , wherein the polyolefin fiber has a sealability of 10 to 12 N for a fiber of 2.2 dtex.
 20. The material according to claim 14 , wherein the material is a non-woven fabric having a weight of 19.5 g/m² and comprising fibers of 2.2 dtex and 40 mm in cut length.
 21. The material according to claim 14 , wherein upon calendaring at a speed of 200 m/min and at a calendaring temperature of about 147 to 149° C., the fabric has a machine direction maximum load at breaking of about 39 N/5 cm. with an elongation of about 44% and a cross direction maximum load at breaking of about 9 N/5 cm. with an elongation of about 86%.
 22. The material according to claim 14 , wherein the polyolefin is polyethylene, polypropylene, polybutene, or polyisoprene.
 23. A method of using an improved sealability polyolefin fiber having an outer surface wherein the polyolefin fiber is in the form of a filament or a cut fiber having a cut length of at least 38 mm and wherein the polyolefin fiber has a count of 1 to 70 dtex and wherein the polyolefin fiber has a heterogeneous structure comprising polyolefin and at least one low-melting polyolefin copolymer wherein the low-melting polyolefin copolymer has a melting temperature of about 140-150° C. and wherein the low-melting polyolefin copolymer comprises at least a portion of the outer surface of the polyolefin fiber, the method comprises incorporating the polyolefin fiber into a non-woven textile material.
 24. The method of using according to claim 23 , wherein the low-melting polyolefin copolymer has a melting temperature of 148° C.
 25. A fiber obtained according to the method of claim 1 , wherein the fiber is used to make a non-woven textile material sealed through a hot calendar, hot air or another system to obtain a non-woven textile material for hygienic and sanitary purposes. 